This invention relates to apparatus to facilitate the placement of shoulder prostheses, and in particular the placement of the glenoid part of shoulder any prostheses.
The replacement of damaged joints by prostheses is a well-established medical treatment to restore the function of a joint. In order to function correctly, and to provide the maximum lifetime for the prosthesis, it must be implanted in an optimum position. Placement in a non-optimal position may result in restricted movement of the joint, an increased rate of wear, and an increased rate of loosening of prostheses. Accurate placement of a prosthesis is therefore a critical part of the joint replacement process.
Partial, total Shoulder or reverse shoulder arthroplasty are effective treatments for damaged shoulder joints and in particular for shoulder arthritis. In a Total Shoulder Arthroplasty both the humeral and glenoid sides of the shoulder joint are replaced by prosthetic components. In a Partial Shoulder Arthroplasty only the humeral side is replaced. A reverse total shoulder arthroplasty, utilised in cases of associated rotator cuff deficiency, both sides of the joint are replaced, but a socket replaces the humeral head and a metaglenoid replaces the glenoid of the scapula.
There is general consensus that the success of total or partial shoulder arthroplasties is dependent on restoring the natural anatomy of the shoulder joint. Significant developments have been made with regard to the humeral side of shoulder prostheses, but recent observations have indicated that loosening rates for glenoid prostheses are very high and that glenoid loosening is the main complication after a total shoulder arthroplasty.
Implanting the glenoid component of a shoulder prosthesis in the optimum position is a difficult process due to, inter alia, limited bone stock of the native glenoid, poor understanding of the optimum position, and a lack of visible landmarks to assist in placement during surgery.
In a typical shoulder arthroplasty to replace the glenoid either supero-lateral or delto-pectoral approaches are made in the patient's outer, upper, arm. The humeral head is cut and the humerus is moved aside to provide a view of the glenoid surface. Only this surface, which is often deformed due to wear, is available as a guide to positioning the prosthetic glenoid and based on this limited information it is difficult for surgeons to accurately locate the optimum position.
Hence, the usefulness of the limited information is further degraded by erosion of the existing glenoid surface due to, for example, arthritis. In such circumstances the surgeon will place the glenoid prosthesis based on the visible surface as if it was in its original anatomical position. The surgeon is forced to estimate the original location based on pre-operative CT-scans, but without operative guidance assistance, this can lead to significant errors in position.
In reverse arthroplasty procedures the positioning of the metaglenoid is known to affect the rate of scapula notches in the resulting joint. Comparable considerations apply to reverse shoulder arthroplasty procedures as to conventional shoulder arthroplasty procedures.
The embodiments described below are not limited to implementations which solve any or all of the disadvantages discussed above.